Controlled Design,Synthesis And Catalytic Activity Of Metal Nanomaterials

Metal nanomaterials have been an active research topic in nanomaterials due to their exellent properties in catalysis,optics,biometrical treatment,electronics and etc.Controlled synthesis of the metal nanomaterials with specific sizes,shapes,compositions and structures by accurately maipulating the experimental parameters is the most effective method for dramatically enhancing their performance.However,it still has substantial uncertainties to explore the key factors,understand the growth mechanism and establish the relationship between their structures and performance during the controlled synthesis of the metal nanomaterials.Thus,we need to further study and explore these uncertainties.The purpose of this dissertation is to explore and design the metal nanomaterials with specific structures,and study the relationship between their structures and performance.In particular,it should be point out that we prefer to reveal the growth mechanism basis on the reaction kinetics.To this end,we have implemented and extended the solution-phase and electrochemical synthetic methods to 9 nm Pt2.4Ni alloy octahedra,Pd@Pt core-shell octahedra,Pd-Pt alloy nanocubes,Pd@Pt1L octahedra,Pt nanocrystals with different high-indexed facets and nickel nanocrystals with predominant {111} orientation.Specifically,Pt-Ni alloy octahedra with different sizes,shapes and compositions were designed,synthesized and characterized,and the effects of preparation conditions on the reaction kinetics were investigated in continuous droplet reactors.We further investigated the effects of Pt-Ni alloy octahedra with different sizes and compositions on the ORR activity and stability.Bimetallic Pd-Pt nanocrystals with different structures were designed,synthesized and the effects of different concentration of KBr on the products structures were investigated.We further quantitativly analysized the reaction kinetics to reveal the growth mechanism of bimetallic nanocrystals with different structures and compared their ORR activities.Pd@Pt1L octahedra with monolayer Pt were designed,synthesized.We further quantitativly analysized the reaction kinetics,shape evolutions and composition changes to reveal the growth mechanism of Pd@Pt1L octahedra with monolayer Pt and investigated their ORR activities.Pt nanocrystals with different high-indexed facets were designed,synthesized and the effects of different kinds of halide ions on the reaction kinetics were investigated.Meanwhile,we also investigated the ORR activities of Pt nanocrystals with different high-indexed facets.Nickel nanocrystals with predominant {111} orientation were designed,synthesized and the effects of voltages on the nucleation and growth kinetics were investigated.We further investigated their activities for methanol oxidation.The main contents and conclusions of this dissertation are listed as follows:1.We reported the use of droplet reactors to achieve the controlled synthesis of 9-nm Pt2.4Ni alloy octahedra and linearly increased the production without compromising the quality and reproducibility.Different from the previous work,the CO gas during the syntheses could be generated in situ through decomposing the W?CO?6 in the solution.We further optimized the concentration of W?CO?6 to ensure a steady flow of uniform droplets in the polytetrafluoroethylene?PTFE?tube,and thus effectively manipulated the sizes and shapes of Pt-Ni nanocrystals.The sizes,shapes and compositions of Pt-Ni nanocrystals could be controlled by optimizing the reaction temperature,the concentration of Ni?acac?2 precursors and the volume ratios of oleylamine and oleic acid solvents.According to these results,we could establish the relationships between different experimental parameters and reaction kinetics.In addition,the structures,surface functional groups,surface compositions and their performances of Pt-Ni alloy octahedra were systematically evaluated.Speciafically,the studies about their performances demonstrated 9-nm Pt2.4Ni alloy octahedra catalyst had a specific activity of 5.74 mA cm-2 at 0.9 V,17 times greater than that of the commerical Pt/C catalyst?0.34 mA cm-2?.The mass activity of 9-nm Pt2.4Ni alloy octahedra catalyst was 2.67 A mg^-1 Pt,11 times higher than that of the commerical Pt/C catalyst(0.24 A mg^-1 Pt)2.We reported the controlled synthesis of Pd@Pt core-shell octahedral and Pd-Pt nanocubes in solution phase and elucidated the formation mechanisms of bimetallic nanocrystals with different structures by quantitatively analyzing the reaction kinetics during the syntheses.In particular,we manipulated the redox potential of the precursors by adding different concentrations of KBr in the solution and thus altered the reaction kinetics for the syntheses of bimetallic nanocrystals with different structures.On the basis of the reaction kinetics constants measured experimentally,we demonstrated the Br ions played an vital role in manipulating the initial reduction kinetics of the precursors.More importantly,it would alter the structures of final products.When the initial reduction rates of the precursors involved in the syntheses were altered,the structures of the products would switch from core-shell to alloy.The studies about their preformances indicated that the mass activity of the Pd@Pt catalyst was 1.05 A mg^-1 Pt,almost 3.1 times as high as that of the Pd-Pt alloy catalyst(0.34 A mg^-1 Pt).After 20000 cycles of accelerated duarability test,the Pd@Pt octahedra still showed a mass activity of more than twice the pristine value of state-of-the-art commerical Pt/C catalyst.3.We reported the controlled syntheses of Pd@Pt core-shell with an atomic layers Pt in solution phase and revealed the formation mechanisms of bimetallic nanocrystals with core-shell structure by quantitatively analyzing the reaction kinetics during the syntheses.We also elucidated that the ratios of the initial reduction rate between two precursors played a vital role in the formation of core-shell structures.According to these results for the formation processes and compositions varieties of the nanocrystals,the Pd precursors would be reduced initially to the formation of Pd cuboctahedra,and then the Pt precursors were reduced to Pt atoms,and selectively deposited on the corners of Pd cuboctahedra,and finally formed a core-shell structure with ultrathin Pt layer through the surface diffusion due to the significant differences between the reduction rates of two precursors.More importantly,we could scale up the syntheses in continuous flow reactors.The studies about their preformances indicated that the mass activity of the Pd@Pt1L catalyst was 0.75 A mg^-1 Pt,almost 55.4 times as high as that of the Pd octahedra catalyst(0.013 A mg^-1 Pt).After 10000 cycles of accelerated duarability test,the Pd@Pt octahedra still showed a mass activity of more than 1.4 times high than the pristine value of state-of-the-art commerical Pt/C catalyst.4.We reported the controlled synthesis of different Pt nanocrystals with highindexed facets in solution phase.We also elucidated that the different kinds of capping agent introduced played a vital role in the formation of high-indexed facets from the aspects of reaction kinetics.We designed a method to manipulate the reduction rate of the precursors by introducing different kinds of halide ions,reaction solvent,and the concentration of reduction agent and thus revealed the formation mechanism.Specifically,different kinds of halide ions introduced in the reaction solution would decrease the reduction rate of the precursors,and these halide ions had different affinities for Pt ions and decreased from iodide ions to chloride ions.Therefore,when we introduced the KI into the reaction solution,the reduction rates of the precursors would dramatically decrease and form the nanocrystals with much bigger sizes.It should be point out that these halide ions would mainly present capping effect and oxidation etching for the nanocrystals when high concentrations of halide ions were introduced.The studies about their preformances indicated that the mass activity of the high-indexed Pt catalyst synthesized with KBr was 0.52 A mg^-1 Pt,almost 1.63 times as high as that of a state-of-the-art commerical Pt/C catalyst.5.We reported the controlled synthesis of mondisperse Ni nanocrystals with predominant {111} orientation by electrochemical methods and elucidated the regulation mechanism for nucleation rates and growth rates in different potential ranges by linear sweep voltammetry,surface morphology and structure analysis.When we altered the deposited voltage ranges,we could control the sizes and shapes of Ni nanocrystals.We further evaluated their performances and established their relationships between crystal structures and catalytic activity.Specifically,the Ni nanocrystals with predominant {111} orientation showed the highest activity with a current densities of 8.47 mA cm-2.The electrochemical impedance spectroscopy further demonstrated Ni{111} with high density of neighboring atoms could provide more free electrons involved in the redox reaction and accelerate the oxidation of the intermediates.